Re: [PATCH v2] hw/riscv: virt: Assume M-mode FW in pflash0 only when "-bios none"

[Sunil V L]

On Mon, May 08, 2023 at 07:37:23AM +0200, Heinrich Schuchardt wrote:
> On 4/25/23 12:25, Sunil V L wrote:
> > Currently, virt machine supports two pflash instances each with
> > 32MB size. However, the first pflash is always assumed to
> > contain M-mode firmware and reset vector is set to this if
> > enabled. Hence, for S-mode payloads like EDK2, only one pflash
> > instance is available for use. This means both code and NV variables
> > of EDK2 will need to use the same pflash.
> > 
> > The OS distros keep the EDK2 FW code as readonly. When non-volatile
> > variables also need to share the same pflash, it is not possible
> > to keep it as readonly since variables need write access.
> > 
> > To resolve this issue, the code and NV variables need to be separated.
> > But in that case we need an extra flash. Hence, modify the convention
> > such that pflash0 will contain the M-mode FW only when "-bios none"
> > option is used. Otherwise, pflash0 will contain the S-mode payload FW.
> > This enables both pflash instances available for EDK2 use.
> > 
> > Example usage:
> > 1) pflash0 containing M-mode FW
> > qemu-system-riscv64 -bios none -pflash <mmode_fw> -machine virt
> > or
> > qemu-system-riscv64 -bios none \
> > -drive file=<mmode_fw>,if=pflash,format=raw,unit=0 -machine virt
> > 
> > 2) pflash0 containing S-mode payload like EDK2
> > qemu-system-riscv64 -pflash <smode_fw_vars> -pflash <smode_fw_code> 
> > -machine  virt
> > or
> > qemu-system-riscv64 -bios <opensbi_fw> \
> > -pflash <smode_fw_vars> \
> > -pflash <smode_fw_code> \
> 
> On amd64 and arm64 unit=0 is used for code and unit=1 is used for variables.
> Shouldn't riscv64 do the same?
> 
Hi Heinrich,

Is that a requirement from distros perspective? That was my original v1
design.

But the reason why I kept unit0 for variables, it helps in keeping current
EDK2 usage model work. Otherwise, current EDK2 will break if we change
the code to unit 0.

Second, since unit 0 for RISC-V is currently assumed to start in M-mode fw
which is secure, I think it makes sense to keep variables also in unit
0.

Thanks!
Sunil